If you buy a regular old book, CD or DVD, you can turn around and loan it to a friend, or sell it…
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Slate's Farhad Manjoo points out more reasons (bothered from Harvard law professor Jonathan Zittrain) why we should be absolutely terrified of "tethered" devices like the Kindle, especially if we're headed toward a truly paperless and discless future, where books, movies and music are all downloaded. Imagine if there were no paper copies of 1984, and Amazon—or whatever company wins the ereader war—deleted it. Or any other book or film that's been banned at one point. It's much easier, after all, to delete them off of a million devices than to actually pull one thousand paper copies out of people's houses. A possibility that's more, uh, possible with breakthroughs like self-destructing data. (One more reason we'll always need something like BitTorent, more than ever in the future, not less.)

If hypotheticals aren't your thing, take the 2004 TiVo vs. Echostar patent infringement case. When TiVo won, the judge ruled that Dish didn't just have to stop selling infringing DVR boxes, they had to actually remotely kill the boxes they'd already sold. Boxes they had installed in people's houses.